Document FR-A-2 851 040 discloses a hemispherical resonator having a vibrating member in the form of a bell fastened by a stem to a base that has main electrodes facing the edge of the bell, and a guard electrode adjacent to the main electrodes.
The main electrodes serve firstly to cause the bell to be set into vibration by applying at least one alternating voltage to the main electrodes while maintaining the bell at a constant potential, and secondly to detect vibration of the bell by picking up a detection signal from the main electrodes.
The guard electrode may be used either for its usual function by connecting it to ground, thereby serving to reduce crosstalk between the electrodes, or it may be used as a control and/or detection electrode by applying appropriate signals to each of the portions of the guard electrode.
In such embodiments, the bell is made of silica, which acts as an electrical insulator. The inside surface and the edge of the bell, and the stem are covered in a layer of metal.
Document US-A-2003/0019296 discloses a resonator in form of a bell of silica having an inside surface and an outside surface both fully covered by a conductive metal coating.
Since the metal plating close to the edge of the bell tends to damp its vibration, thereby worsening drift, proposals have been made to leave the silica bare on the outside surface of the bell.
It is known for example from document U.S. Pat. No. 6,474,161 a hemispherical resonator comprising a vibrating member in the form of a bell of silica partially covered by a conductive metal coating. The conductive metal coating covers a plane annular edge of the bell and forms conductive tracks extending on an inside surface of the bell between a pole of the resonator and said edge. The outside surface of the bell is left completely uncovered.
It is also know from document US-A-2004/0154396 such a resonator in which the inside surface of the bell is fully covered by the conductive metal coating.
Nevertheless, it has been found that leaving the silica bare on the outside surface of the bell adjacent to the edge thereof gives rise to a modification in the looked-for electrical image and thus gives rise to an error in angle measurement. This modification comes from field lines that move between the electrodes and the metal-plated and biased zones of the resonator by passing through the silica, thereby causing charge to migrate very slowly in the silica which is not infinitely insulating, and until potential equilibrium is reached. Consequently, the field lines and thus the electrostatic efficiency are transformed over time. Such variation over time is very difficult to correct by calculation.